1. Field of the Invention
Embodiments of the preset invention relate to a measurement technique to measure neutrons generated in a nuclear fission reactor and a nuclear fusion reactor.
2. Description of the Related Art
Neutrons generated in a nuclear fission reactor of a light-water reactor in a commercial nuclear power plant are measured by use of fission counter tubes because of their excellent discrimination performance to discriminate neutrons from gamma rays. In a state of a low reactor power, output signals from the fission counter tubes are measured as pulse signals. In a state of a relatively high reactor power, the output signals from the fission counter tubes cannot be individually measured because the pulse signals are overlapped with each other. For this reason, the neutrons are measured with the Campbell's method of utilizing statistical fluctuations of detector output signals (for example Japanese Patent Laid-Open No. 59-21111).
In nuclear fusion reactors, duration time of nuclear fusion reactions of heavy hydrogen (D-D reaction) has been enhanced thanks to recent technical developments, and the number of neutrons generated by the D-D reaction has been increased. Hence, in the measurement of neutrons generated from a nuclear fusion reactor using fission counter tubes, it has been required to use the measurement domain of the Campbell's method beyond the pulse measurement domain.
It has been well known that accuracy of a measurement result with the Campbell's method is proportional to a time constant of an averaging circuit in an output stage of a measurement apparatus.
Meanwhile, enhancement of performances of microprocessors (MPU) and digital signal processors (DSP) facilitate implementation of digital signal processing technologies to real neutron measurement apparatuses. Particularly, in signal processing using the fast Fourier transform (FFT), it is possible to realize optimum filtering characteristics that cannot be realized by conventional analogue filtering.
In the neutron measurement with the Campbell's method, a root mean square calculation is applied to input signals so as to calculate statistical fluctuations of detector output signals, and thus this neutron measurement is often subjected to influences of noise signals if the noise signals are overlapped with the input signals.
Recently, in order to prevent influences on neutron measurement apparatuses of high-frequency noises of approximately 1 MHz, for example, which are generated by inverters that have recently been used in power source apparatuses or electric motors, such noise-prevention is required for neutron measurement apparatuses that reinforce shields of their measurement systems, or provide ferrite cores in their noise propagating paths, for example.
In conventional neutron measurement apparatuses, a signal processing circuit for processing detector output signals (analogue signals) from a neutron detector 1 is provided with a preamplifier 2, an AC (alternating current) amplifier 3 and an analogue filter device 4, a square calculating circuit 5, and a time constant circuit 6, so as to apply filtering to the input signals and the output signals for the noise prevention, as shown in FIG. 8.
In conventional analogue filters, infinite impulse response (IIR) filters or finite impulse response (FIR) filters that perform digital filtering in the time domain, it is impossible to realize perfect filtering characteristic, and it is difficult to completely remove influences of noises.